1. Why can't regular electric motors be used in high-altitude areas?
Altitude negatively impacts motor temperature rise, motor corona (in high-voltage motors), and commutation in DC motors. The following three aspects should be considered:
(1) The higher the altitude, the greater the temperature rise of the motor and the lower the output power. However, when the temperature decreases sufficiently with increasing altitude to compensate for the effect of altitude on temperature rise, the rated output power of the motor can remain unchanged;
(2) Anti-corona measures should be taken when high-voltage motors are used at high altitudes;
(3) Altitude is detrimental to the commutation of DC motors, so attention should be paid to the selection of carbon brush materials.
2. Why does a motor generate shaft current?
The current in the motor's shaft-bearing housing-base circuit is called shaft current.
reason:
(1) Magnetic field asymmetry;
(2) There are harmonics in the power supply current;
(3) Poor manufacturing and installation, resulting in uneven air gap due to rotor eccentricity;
(4) There is a gap between the two semicircles of the detachable stator core;
(5) The number of stator core pieces stacked in a fan shape is not appropriate.
harm:
This causes erosion on the surface or balls of the motor bearing, forming pinholes, which deteriorates the bearing's operating performance, increases friction loss and heat generation, and ultimately leads to the bearing burning out.
prevention:
(1) Eliminate pulsating magnetic flux and power supply harmonics (e.g., by adding an AC reactor to the output side of the frequency converter).
(2) When designing the motor, the bearing housing and base of the sliding bearing are insulated, and the outer ring and end cover of the rolling bearing are insulated.
3. Why is it not advisable for motors to operate under light loads?
When a motor is running under light load, it will cause:
(1) The motor has a low power factor;
(2) The motor has low efficiency.
This will result in equipment waste and uneconomical operation.
4. Why can't a motor start in a cold environment?
If a motor is kept in a low-temperature environment for an extended period of time, it will:
(1) Cracking of motor insulation;
(2) The bearing grease has frozen;
(3) The solder of the wire joint is powdered.
Therefore, motors should be heated and stored in cold environments, and the windings and bearings should be inspected before operation.
5. Why can't a 60Hz motor use a 50Hz power supply?
In motor design, silicon steel sheets are generally designed to operate in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux and the excitation current, which will lead to an increase in motor current and copper loss, ultimately resulting in a higher motor temperature. In severe cases, the motor may even burn out due to overheating of the coil.
6. Does clicking "soft start" save energy?
Soft starters have limited energy-saving effects, but they can reduce the impact on the power grid during startup and achieve smooth startup, protecting the generator set. According to the law of conservation of energy, due to the addition of relatively complex control circuits, soft starters not only do not save energy but also increase energy consumption. However, they can reduce the starting current of the circuit, thus playing a protective role.
7. Why do residual current circuit breakers (RCCBs) tend to trip when used with frequency converters?
This is because the output waveform of the frequency converter contains high-order harmonics, and leakage current will be generated in the motor and the cable between the frequency converter and the motor. This leakage current is much larger than that when the motor is driven by the mains frequency, so this phenomenon occurs.
